There is no "perfect ratio". It really depends on what you ride. Most people start from a 2:1 ratio an work from there. Try it out by not switching gears during a whole ride. Find the ratio you can do almost anything on(because there is no ratio you can do everything on). What bike do you own? Sometimes there are "magic ratios" that allow you to run an 8 or 9-speed chain without the use of a tensionner or horizontal drop outs.,

Most of the current micro drive BMX bikes work out to roughly 55. It's a good place to work from.

36/17x26=55.1 This is what I originally rode on my first 26" single speed. I moved up to 36/16 once I started riding the local downhill BMX track. Works out to 58.

Since you have gears right now start playing with them. Find out your chainrings' and cogs sets' tooth counts. Ride. Try to find which gear lets you do the most riding without needing to shift. Use a calculator and find equivalents. Singlespeed your bike. Ride more.